1. Field of the Invention
The invention relates to a pelletized organopolysiloxanes, to a process for their preparation, and to their use as an additive for thermoplastics.
2. Background Art
Silicone-based monomers and polymers are increasingly used in the plastics industry, from polymer preparation through polymer compounding and final product manufacture. By way of example, low-molecular-weight organopolysiloxanes with viscosities up to 1000 mm2/s are widely used in the plastics industry as external mold-release agents. Relatively high-molecular-weight organopolysiloxanes in the viscosity range from 10,000 to 100,000 mm2/s have been used for about 30 years as internal additives in thermoplastic polymer to provide processing advantages and improvements in surface properties, among which, by way of example, are low coefficient of friction, better slip properties, better abrasion resistance, better wear values, better scratch resistance and surface gloss, better demoldability, lower rejection rate, easier processing, better throughput, lower energy consumption during processing, and faster cycle times in shaping processes.
In addition, organopolysiloxanes have better stability than conventional processing aids and lubricants, for example, waxes and fatty acid derivatives. Ultrahigh-molecular-weight organopolysiloxanes with viscosities of from 1,000,000 to 100,000,000 mm2/s have additive properties comparable to those of relatively high-molecular-weight organopolysiloxanes, generally with better effectiveness and higher efficiency. This improved property profile is attributed to a higher level of mechanical interaction between the long-chain ultrahigh-molecular-weight organopolysiloxanes and the polymer chains of the thermoplastics, and results in greater permanency, reduced migration, and better printability, adhesive properties, and weldability.
However, liquid organopolysiloxane additives, especially those with relatively high viscosities, have the disadvantage that their introduction into thermoplastic melts during the extrusion process is difficult, and in general, only achievable by use of specialized technical equipment. Extrusion processes preferably use solid constituents, most preferably in the form of conventional pelletized material. A disadvantage of solid constituents in powder form is that it is more difficult to manufacture homogeneous premixes in the form of dry blends from plastics pellets and pulverulent additives, and that such premixes, even when used, have greater susceptibility to segregation during storage and metering than premixes manufactured from two or more different pelletized materials. Segregation and irregular metering are problematic especially when pulverulent additives, the amounts of which are generally very small, are blended with pelletized polymer material. Non-uniform metering leads to inhomogeneous product quality, which is undesirable. For this reason, concentrates or masterbatches have been developed comprising relatively high-viscosity and ultrahigh-viscosity liquid organopolysiloxanes in the form of pellets. However, these organopolysiloxane concentrates have a number of disadvantages. For example, the organopolysiloxane content ranges only from 20% to a maximum of 50%, the remainder being a thermoplastic carrier material. The presence of such carrier materials limits the range of applications of the pelletized material, because the carrier has to be matched to the particular plastic to which the concentrate is added. In view of the wide variety of thermoplastics, this consequently requires providing a wide variety of concentrates of organopolysiloxane in different thermoplastic carriers. This is especially disadvantageous for plastics compounders or final processors who utilize a wide variety of different plastics and consequently have to inventory a correspondingly wide variety of organopolysiloxane concentrates.
Organopolysiloxane powders do not have the disadvantage of incorporation of different carrier materials. U.S. Pat. No. 5,346,941 describes pulverulent compositions comprising a liquid organopolysiloxane, silica, and a specific silicone resin, and its use for modifying thermoplastics. U.S. Pat. No. 5,391,594 describes compositions comprising a plastic and a silicone powder with a particle size of from 1 to 1000 μm which includes a polydiorganosiloxane and silica. However, pulverulent organopolysiloxane additives have the disadvantage, described at the outset, of segregation and inhomogeneity with pelletized materials. In the case of additives employed in small amounts, this is particularly undesirable, resulting in variable product quality.